D,L-thioctic acid is used in the form of the racemic mixture as a pharmaceutical preparation for treating acute and chronic liver diseases as well as poisonings. The optically active R-(+)-.alpha.-lipoic acid is a natural substance which occurs in a slight concentration in animals and humans. R-(+)-.alpha.-lipoic acid acts as coenzyme in the oxidative decarboxylation of .alpha.-keto acids. The R-configuration of the naturally occurring .alpha.(+)-lipoic acid was confirmed by an enantioselective synthesis of the (-) antipode starting from S-malonic acid (M. H. Brookes, B. T. Golding, D. A. Howes, A. T. Hudson, "J. Chem. Soc.", Chem. Commun. 1983, p. 1051).
There are indications that the two enantiomeric forms of .alpha.-lipoic acid do not exhibit the same biological activity but rather that the S(-) enantiomer exhibits a lesser activity (J. C. Gunsalus, L. S. Barton, W. Gruber, "J. Am. Chem. Soc." 78, p. 1763, 1956). It is therefore necessary for a rational pharmaceutical therapy to use the more active enantiomeric form of .alpha.-lipoic acid. The known methods of producing .alpha.-lipoic acid yield only a racemic mixture (see DE-OS No. 35 12 911.5 and the literature cited there). A method for the resolution of racemates with D(-)arabinose is known (L. G. Chebotareva, A. M. Yurkerisch, "Khim.-Farm. Zh." 14(9), pp. 92-99, 1980; "C.A." 94 (13), 103 722 g); however, the yields achieved are only slight and the method is uneconomical.
The first known asymmetric synthesis for producing R(+)-.alpha.-lipoic acid starts from a 7-stage reaction sequence (J. D. Elliott, J. Steele, W. S. Johnson, "Tetrahedron Lett.", 1985 Vol. 26, p. 2535). Expensive chiral 2,4-pentane diol must be used as the starting material in this synthesis, so that an economical route to enantiomerically pure .alpha.-lipoic acid is not achieved. Furthermore, only one enantiomeric form can be produced.